1. Field of the Invention
The present invention relates to a wideband wireless mobile communication system supporting SFBC (Space Frequency Block Coding).
2. Discussion of the Background Art
Fading is the distortion that a carrier-modulated telecommunication signal experiences over certain propagation media. A fading channel is a communication channel that experiences fading. In wireless systems, fading is due to multi-path propagation and is sometimes referred to as multipath induced fading.
In wireless communications, the presence of reflectors in the environment surrounding a transmitter and receiver creates multiple paths that a transmitted signal can traverse. As a result, the receiver sees the superposition of multiple copies of the transmitted signal, each traversing a different path. Each signal copy will experience differences in attenuation, delay and phase shift while traveling from the source to the receiver. This can result in either constructive or destructive interference, amplifying or attenuating the signal power seen at the receiver. Strong destructive interference is frequently referred to as a deep fade and may result in temporary failure of communication due to a severe drop in the channel signal-to-noise ratio.
In telecommunications, a diversity scheme refers to a method for improving the reliability of a message signal by utilizing two or more communication channels with different characteristics. Diversity plays an important role in combating fading and co-channel interference and avoiding error bursts. Diversity exists because individual channels experience different levels of fading and interference. Multiple versions of the same signal may be transmitted and/or received and combined in the receiver. Alternatively, a redundant forward error correction code may be added and different parts of the message transmitted over different channels. Diversity techniques may exploit the multipath propagation, resulting in a diversity gain, often measured in decibels.
Diversity scheme can be classified into time diversity, frequency diversity, space diversity, polarization diversity, multi-user diversity, and cooperative diversity. For time diversity among these, multiple versions of the same signal are transmitted at different time instants. Alternatively, a redundant forward error correction code is added and the message is spread in time by means of bit-interleaving before it is transmitted. Thus, error bursts are avoided, which simplifies the error correction. For frequency diversity, the signal is transferred using several frequency channels or spread over a wide spectrum that is affected by frequency-selective fading.
In a broadband wireless mobile communication system, resources can be allocated in distributed manner for transmission to have frequency diversity gain. Strategies for distributed allocation of resources can be different according to the combinations of the number of DRUs (Distributed Resource Units) assigned to a user and the available bandwidth for forming DRUs for the user. The number of DRUs assigned to a user is proportional to the packet size allocated to the user, and the available bandwidth for forming DRUs is proportional to the number of LRUs (Logical Resource Units) allocated to the user.
FIG. 1 illustrates possible combinations of a packet size and the number of LRUs (the available bandwidth) for forming DRUs.
Region 1 of FIG. 1 represents the combination of small amount of available bandwidth and large packet size, and Region 3 represents the combination of large amount of available bandwidth and large packet size. In Region 1 and Region 3, performance difference between possible distributed resource allocation strategies is negligible because the packet size is large in these regions so that the packet is more likely to spread over frequency.
However, even in region 4, performance difference between possible distributed resource allocation strategies would not be significant if the size of a fractional PRU (Physical Resource Unit) or MRU (Mini physical Resource Unit) is small, because a number of MRUs of small size can be allocated in the manner that the MRUs spread over frequency axis due to large available bandwidth for forming DRUs. Therefore, in terms of diversity gain, the smaller a MRU size is, the better a system performance becomes. Therefore, generally one sub-carrier as the minimum unit for forming the DRU can obtain more diversity gain than other structure for the minimum unit.
However, designing a MRU as a minimum unit for forming a DRU should be approached in view of flexibility as well as diversity because a wireless mobile communication system may support various sub-frame configurations. For example, a communication system may adopt FFR (Fractional Frequency Reuse) and FDM (Frequency Division Multiplexing) of DRU and CRU (contiguous resource unit). Also in some configuration, there exist those sub-frame configurations where STBC (Space-Time Block Code) is not suitable for data transmission. STBC is not suitable for a sub-frame having “odd” number of symbols. In TDD (Time Division Duplexing) mode, a total of odd number of symbols may be allocated for an irregular sub-frames (5 symbols) for TTG (Transmission Transition Gap), for a sub-frame including preamble, for a sub-frame including mid-amble, for an irregular sub-frame with other CP (Cyclic Prefix) size (e.g., 7 symbols for 1/16 CP), for a sub-frame including TDM MAP (Time Division Multiplexing), etc. In FDD (Frequency Division Duplexing) mode, a total of odd number of symbols may be allocated for a sub-frame including preamble, a sub-frame including mid-amble, an irregular sub-frame with other CP size (e.g., 7 symbols for 1/16 CP), a sub-frame including TDM MAP, etc.
Although STBC is not suitable for many sub-frame configurations, SFBC (Spatial Frequency Block Coding) can support all the sub-frame configurations. Therefore, as discovered by the present inventors, a need has arisen to create a structure for the minimum unit for forming a DRU for replacing STBC by SFBC or to support both STBC and SFBC, in consideration of diversity gain performance.